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Excess and Limiting Reagents

Key Terms and concepts

Stoichiometric mixture
Excess reagent
Limiting reagent

Skills to develop

Excess and Limiting Reagents

Chemical reaction equations give the ideal stoichiometric relationship among reactants and products.

However, the reactants for a reaction in an experiment are not necessarily a stoichiometric mixture. In a chemical reaction, reactants that are not use up when the reaction is finished are called excess reagents. The reagent that is completely used up or reacted is called the limiting reagent, because its quantity limit the amount of products formed.

Let us consider the reaction between sodium and chlorine. The reaction can be represented by the equation:

2 Na + Cl2 = 2 NaCl,

It represents a reaction of a metal and a diatomic gas chlorine. This balanced reaction equation indicates that two Na atoms would react with two Cl atoms or one Cl2 molecule. Thus, if you have 6 Na atoms, 3 Cl2 molecules will be required. If there is an excess number of Cl2 molecules, they will remain unreacted. We can also state that 6 moles of sodium will require 3 moles of Cl2 gas. If there are more than 3 moles of Cl2 gas, some will remain as an excess reagent, and the sodium is a limiting reagent. It limits the amount of the product that can be formed.

Chemical reactions with stoichiometric amounts of reactants has no limiting or excess reagents.

Example 1

Calculate the number of moles of CO2 formed in the combustion of ethane C2H6 in a process when 35.0 mol of O2 is consumed.
The reaction is
2 C2H6 + 7 O2 = 4 CO2 + 6 H2O
            4 mol CO2
35.0 mol O2 ---------- = 20.0 mol CO2
            7 mol O2
A balanced equation for the reaction is a basic requirement for identify the limiting reagent even if amounts of reactants are known.

Example 2

Two moles of Mg and five moles of O2 are placed in a reaction vessel, and then the Mg is ignited according to the reaction
Mg + O2 = MgO. Balance this equation and identify the limiting reagent in this experiment.

The balanced reaction is, 2 Mg + O2 = 2 MgO Thus, two moles of Mg require only ONE mole of O2. Four moles of oxygen will remain unreacted. Oxygen is the excess reagent, and Mg is the limiting reagent.
Answer these questions:
How many moles of MgO is formed?
What is the weight of MgO formed?

Skill Developing Problems

  1. Which of the following will react most vigorously with Cl2?
    Li, Na, K, Fe, Al, Mg, or Ar?

    Skill -
    Predict reactivity from location of element in the periodic table.

  2. One mole of Cl2 molecules would react with how many moles of Na metal?
    Skill -
    Write a reaction equation for a chemical reaction.

  3. At room temperature (25 °C) what is the state of sodium, solid, gas or liquid?
    Skill -
    Know where to look up properties of elements or substances.

  4. Which of the following is the best conductor? sodium metal, solid NaCl, chlorine gas, dilute NaCl solution, or molten NaCl?

  5. What positive ions are present in solutions of NaCl?

  6. If you put equal weights of sodium metal and chlorine gas into a reaction vessel, which is the limiting reagent?
    Skill -
    Identify excess and limiting reagent.

  7. Equal weights of H2 and O2 are placed in a balloon and then ignited. Assume reaction goes to completion, which gas is the excess reagent?

  8. Equal weights of H2 and F2 are placed in a balloon and then ignited. Assume reaction goes to completion, which gas is the limiting reagent?
    Skill -
    Identify the excess and limiting reagent.

  9. The reaction:
    2 Al (s) + Fe2O3 (s) = 2 Fe(liquid) + Al2O3
    takes place in the thermite mixture when it is ignited by a magnesium ribbon. A thermite mixture contains a mass ratio of 1 to 2 for Al and Fe2O3. Which one is the limiting reagent?

    Discussion -
    A stoichiometric mixture has a mass ratio of 54:160 (nearly 1:3) for Al:Fe2O3.